It has well known that polystyrene foam polymers are particularly sensitive to the ignition with flame, and this is the reason why they need a suitable protection, in terms of flame retardant properties. According to the prior art, the flame retardant property is achieved by means of addition of at least an organic halogenated compound, particularly a brominated compound, such as hexabromocyclododecane, with or without other additives such as, for example, antacids, dripping promoters, nucleators.
Hexabromocyclododecane is, however, under public and regulatory pressure, due to its intrinsic characteristics, to the possibility that its use could cause bioaccumulation in the environment, as well as due to the strong pressure which exists in order to avoid the use of halogenated compounds in polymeric compositions.
In view of the above, intensive research is ongoing to find suitable substitute compounds or compositions, which allow to avoid the use of halogen derivatives in flame retarded polymeric compositions.
Generally speaking, the “new generation” of flame retardant compositions, should fulfill some basic requirements, here below schematically indicated:                Flame Retardant (FR) additives should have an halogen content as low as possible for environmental reasons;        FR additives total loading should be as low as possible in order to not affect mechanical properties of the composition;        FR additives should have suitable thermal resistance to be processed safely at temperatures of at least 180° C. or higher, for example up to 250° C.;        FR additives should have low toxicity;        FR additives should be fully colorable, i.e. the color of the final polymeric composition containing the flame retardant agent should be as closed as possible to the same composition in the absence of FR additive.        
It is known that the above requirements can be achieved by using metal inorganic hypophosphites into the polymer blend with or without halogen containing additives. However, when special and particular polymers are considered, the use of metal inorganic hypophosphites as flame retardant additives, does not suffice in order to obtain good and satisfactory flame retardant properties.
In this cases, it could be necessary to introduce at least an additional essential compound into the composition, such as, for example, a radical initiator, also herein called “organic synergic”. This kind of compound could be helpful in order to improve the flame retardant properties of the final polymeric composition.
When the thermoplastic polymer is polystyrene, an aromatic polymer made from the aromatic monomer styrene, it is necessary to face with additional problems in formulating a good flame retarded polystyrene composition. In fact, for example extruded foams of polystyrene (X-PS) are commonly widely used in constructions, for building insulation, and as packing material, and for these reasons their flame retardant characteristics are very relevant.
Usually, due to their use in constructions, it is necessary to improve the flame retardant properties of extruded polystyrene foams with respect to other polymeric composition, and, at the same time, it is also necessary to decrease the amount of halogenated flame retardant agents in the polymeric composition, due to environmental and safety reasons. For the above reasons, it is well known that at least a certain amount of halogenated compound, if not the total of it, could be replaced by a so called “organic synergic”, which is a radical initiator, as well as accompanied by the presence of metal inorganic hypophosphites, as already explained above.
For example, US 2009/0149561 describes polymer foams prepared using 5,5-bis(bromomethyl)-2-oxo-1,3,2-dioxaphosphorinane or brominated 2-oxo-1,3,2-dioxaphosphorinane compounds. The minimum content of bromine in the reported examples are, however, equal to 0.8% by weight on the total percentage of the composition, which is, in any case, a still quite high amount.
WO 2010/083068 teaches a polystyrene foam composition containing at least 0.8% by weight on the total percentage of the composition of bromine and at least 1.5% by weight on the total percentage of the composition of graphite. These compositions also contain a quite relevant amount of halogen compound and are not fully colorable because of the presence of graphite.
WO 2008/039833 discloses polystyrene foam compositions with very low color, containing N, 2-3-dibromopropyl-4,5-dibromohexahydrophthalimide, a flame retardant agent (FR) that does not tend to degrade when processed. However, the preferred concentration of the FR agent is between 3% and 4% by weight on the total percentage of the composition and, more particularly 3.5% by weight on the total percentage of the composition, thus corresponding to a bromine content of 2.2% by weight on the total percentage of the composition.
US 2008/0058435 discloses polystyrene foam compositions free from halogen or with low amounts of halogen, containing 9,10-dihydro-9-oxa-phosphenanthrene 10 oxide (DOP) and its derivatives, hydrolysis products or metal salts. This compound is an organic phosphinate, with the oxidation state of phosphorus being formally in the +1 value.
Thermoplastic molding compositions containing a polyolefin polymer, particularly polypropylene, at least an inorganic hypophosphite and an halogen containing flame retardant agent as synergistic mixture, are described in WO 2007/010318. The disclosed composition also contains an organic compound, able to form free radicals during flaming. This compound was added to the disclosed composition in order to improve flame retardant properties of the polymeric composition. Despite the fact that WO 2007/010318 discloses in general terms polyolefin polymers (and explicitly mentions styrene, methyl styrene and corresponding copolymers) as suitable for being flame retarded as compositions containing at least an inorganic hypophosphite and an halogen containing flame retardant agent as synergistic mixture, the experimental part and all the evidences of an effective flame retardant characteristic are indicated only when polypropylene is chosen as polyolefinic polymer. No example is included for different polyolefins.